RO membranes have been used in ultrapure water production plants, wastewater recovery plants, seawater desalination plants, and the like and can remove most of organic substances, inorganic substances, and the like contained in water.
A rejection of a permeable membrane including an RO membrane, to substances such as inorganic electrolytes and water soluble organic substances, is decreased by degradation of a high molecular weight base material due to influences of oxidizing substances, reducing substances, and the like present in water and other causes, and as a result, required treated water quality may not be obtained in some cases. This degradation may gradually occur during long-term use or may suddenly occur by an accident in some cases. In some cases, the rejection of a fresh permeable membrane may not satisfy a level required as a product.
In a permeable membrane system using an RO membrane or the like, raw water is treated in a pre-treatment step using chlorine (such as sodium hypochlorite) in order to prevent biofouling caused by slime on a membrane surface. However, when water containing residual chlorine at a high concentration is supplied to a permeable membrane, the permeable membrane is degraded since chlorine has a strong oxidizing action.
In order to decompose residual chlorine in water to be treated, a reducing agent such as sodium bisulfite is sometimes added to water to be treated. However, when a metal such as Cu and/or Co is contained in the water to be treated, even if a large amount of sodium bisulfite is added to the water, an RO membrane is degraded (Patent Document 1 and Non-Patent Document 1). When the permeable membrane is degraded, the rejection thereof is decreased.
The following methods have been proposed as a method for improving a rejection of a permeable membrane such as an RO membrane:
i) A method for improving a rejection of a permeable membrane by adhesion of an anionic or a cationic high molecular weight compound to a membrane surface has been disclosed (Patent Document 2).
In the method described above, an effect of improving the rejection of a degraded membrane is not sufficient.
ii) A method for improving a rejection of a nanofiltration membrane or an RO membrane by adhesion of a compound having a poly(alkylene glycol) chain to a membrane surface has been disclosed (Patent Document 3).
This method is also not a method which sufficiently improves the rejection of a degraded membrane without remarkably reducing a permeation flux.
iii) A method for preventing membrane contamination and/or degradation in quality of permeated water has been disclosed in which a treatment using a nonionic surfactant is performed on a nanofiltration membrane or an RO membrane having an increased permeation flux and an anionic charge to reduce the permeation flux to an appropriate range (Patent Document 4). In this method, the nonionic surfactant is brought into contact with and is adhered to a membrane surface so as to set the permeation flux to a range of +20% to −20% of that at the start of use.
In order to improve a rejection of a seriously degraded membrane (membrane having a salt rejection decreased to 95% or less) by this method, a considerable amount of the surfactant is required to be adhered to the membrane surface, and as a result, a remarkable decrease in permeation flux may occur in some cases. One example of the above Patent Document 4 has disclosed that an aromatic polyamide RO membrane having a permeation flux of 1.20 m3/m2·day, a NaCl rejection of 99.7%, and a silica rejection of 99.5% as the initial performance at a production stage is used for 2 years, and the membrane thus obtained is used as an oxidation degraded membrane. In Patent Document 4, a membrane having a NaCl rejection of 99.5% and a silica rejection of 98.0%, which is not so much degraded, is used as an object, and it has not been disclosed that by the method described above, the rejection of a degraded permeable membrane is sufficiently improved.
iv) A method for improving the salt rejection by adhesion of a tannic acid or the like to a degraded membrane has been disclosed (Non-Patent Document 2).
However, an effect of improving the rejection obtained by this method is not significant. For example, even when the salt rejection of a degraded RO membrane, ES20 (manufactured by Nitto Denko Corp.) or SUL-G20F (manufactured by Toray Industries, Inc.), is improved by this method, a solute concentration of permeated water through the membrane after the improvement cannot be decreased to ½ of that of permeated water through the membrane before the improvement.
v) A method for improving a rejection of an RO membrane by addition of a poly(vinyl methyl ether) (PVME) to a tannic acid has been disclosed (Non-Patent Document 5). In this method, the concentration of the chemical agent to be used is relatively high, such as 10 ppm or more. In addition, when the membrane is treated by this method, the permeation flux of the membrane is decreased by approximately 20%. Furthermore, the rejection may be hardly improved in some cases.
Non-Patent Documents 3 and 4 have disclosed that in a polyamide membrane degraded by an oxidizing agent, the C—N bond of the polyamide linkage of a membrane base material is broken, and hence an inherent sieve structure of the membrane is destroyed.
The related rejection improving methods described above have the following problems a to c.
a) Since a substance is newly adhered to the surface of the permeable membrane, the permeation flux thereof is reduced. For example, when a degraded membrane is treated by a rejection improving treatment so that the solute concentration of water permeated through the membrane which is treated by a rejection recovery treatment is decreased to ½ of that of water permeated through the membrane which is not yet treated by the recovery treatment, in some cases, the permeation flux may be considerably decreased by 20% or more as compared to that before the treatment is performed.
b) When a chemical agent having a high concentration is added, TOC of brine separated by the membrane is increased. In addition, it is not easy to restore the membrane while water to be treated is supplied through the membrane and is collected.
c) For a membrane which is seriously degraded, the rejection thereof is not easily recovered.